Asteroid 4179 Toutatis: 1996 Radar Observations

Abstract

We report initial results of daily delay/Doppler observations of Toutatis with the Goldstone 8510-MHz (3.5-cm) radar during Nov. 25–Dec. 3, 1996. Using the physical model of Toutatis derived from 1992 radar observations (Hudson and Ostro 1995,Science270, 84–86) to analyze the new data, we obtain refined estimates of the asteroid's orbit, spin state, and surface properties. The asteroid's centimeter-to-decameter surface characteristics are strikingly uniform. The disc-integrated circular polarization (SC/OC) ratio μCaverages 0.29±0.01 and is independent of rotational orientation at the several percent level. Dual-polarization images reveal a slight drop in μCat echo leading edges, which we interpret as the signature of a smooth surface component. The OC radar albedo averages 0.24±0.03; it depends on rotational orientation, as expected from the asteroid's angular scattering behavior (limb-darkening slightly more than Lambertian). The OC albedo of a sphere with Toutatis' radar properties would be 0.21, or three times the lunar value. The radar properties and available nonradar constraints are consistent with Toutatis' surface having a smooth component that is at least 1/3 covered by rocks at least as large as the wavelength. If this S-class asteroid is mineralogically similar to stony–iron meteorites, then the smooth surface component probably is regolith whose porosity resembles that of lunar soil. If the mineralogy is ordinary chondritic, then the smooth surface component is probably solid with not much more than a centimeter of overlying regolith. We report delay-Doppler astrometry referenced to the asteroid's center of mass (COM) for each day of our experiment. An orbit solution that incorporates those measurements as well as the radar astrometry reported by Ostroet al.(1995,Science270, 80–83) and 588 optical astrometric observations from 1988 through March 1997 has weighted rms residuals of 0.98 arcs, 0.10 Hz (1.8 mm s−1in radial velocity), and 0.49 μs in time delay (73 m in range). Integration of that orbit into the past and future shows that Toutatis' pattern of close approaches to Venus, Earth, and Mars is highly asymmetric about the current epoch. The probability of the orbit intersecting Earth is zero for at least the next six centuries. Toutatis will make its closest planetary approach since at least 1353 and until at least 2562 on Sep. 29, 2004, when the closest COM-to-COM separation of Earth and Toutatis will be 1,549,834±10 km (4.0 lunar distances). We use refined spin-state parameters and the 1995 shape model to generate “movies” that predict the asteroid's rotational motion during its 2004 close approach, in geocentric and inertial frames.